It means nothing at all and nobody can estimate an ETA on your decision without knowing the specific school. I went under review within a week of submitting at several schools. A few I heard back from within two weeks and a few I'm still waiting on two months later. In some offices, under review means an admissions committee member is actually reviewing your app at that time and at others it simply means that your app was thrown in a pile to be reviewed a month or so later.